The investigation found that slave labor is producing a basic yet essential part of the shrimp industry. Slaves -- who are reportedly beaten, chained, tortured and sometimes killed by their captors -- are forced to haul up huge "trash fish" harvests. According to the report, those fish are then ground to fishmeal and sold to CP Foods, a giant Thai food conglomerate which uses the meal to feed its farmed shrimp. Those shrimp are sold to Walmart, Costco and U.K. retailers such as Tesco, the Guardian found.

Walmart said that it's taking action in response to the revelations.

"We are actively engaged in this issue. Walmart is playing an important role in bringing together stakeholders, NGOs and other private sector companies to help eradicate human trafficking from Thailand's seafood export business," the company said in a statement emailed to CBS MoneyWatch.

Costco didn't immediately respond to a request for comment. Costco told USA Today that it is "working with our suppliers of Thai shrimp to require them to take corrective action to police their feedstock sources with respect to poor labor practices." That has involved visits from Costco's buying staff and discussions with the Thai government, suppliers and others in the industry.

CP Foods, which buys the slave-linked fishmeal, said in a statement sent to CBS MoneyWatch that "the issue of endemic slavery in Thailand's seafood supply chain affects all producers" in that country. The company notes on its website that its food products are consumed by 3 billion people across the world.

"As purchasers of fishmeal, CP Foods has been actively working to solve this problem since April 2013 and will continue to do so," the company said. "We are now in the process of auditing our entire operation so that we can introduce an independent spot-check system across our supply chain to ensure it is and continues to be slavery free."

More than a dozen migrant workers from Asian countries such as Cambodia told the Guardian that they had paid brokers to help them find work in Thailand. Instead, they were sold to boat captains for a little over $400 each.

One former monk told the publication, "I thought I was going to die. They kept me chained up, they didn't care about me or give me any food ... They sold us like animals, but we are not animals -- we are human beings."

Others told the Guardian that they had witnessed slaves being killed by their captors.

Because of weak government policies and corruption, there are likely hundreds of thousands of workers in forced labor situations in Thailand, according to Anti-Slavery International.